Insolation-driven northern Atlantic westerly wind patterns shaped the mid-Pleistocene transition in three phases

The Mid-Pleistocene Transition ( ~ 1200–650 ka) marked a shift to 100-kyr glacial cycles, despite no significant change in insolation cyclicity. In this study, a high-resolution pollen record from the southwestern Iberian margin, combined with climate modeling, reveals a three-phase climate evolution linked to boreal summer insolation amplitude. In the early phase ( ~ 1200–930 ka), strong insolation minima shifted westerlies north, boosting ice accumulation, while maxima promoted melting, limiting ice growth. The middle phase ( ~ 930–790 ka) followed regoliths loss and featured weaker insolation extremes, leading to the southward migration of the westerlies, mega-droughts and greater ice buildup. In the late phase ( ~ 790–680 ka), stronger minima pushed westerlies north, aligning with longer, colder glacials. These findings suggest that changes in insolation amplitude drove shifts in westerly wind patterns, influencing hydroclimate and ice sheet development, and may have contributed to the transition to 100-kyr glacial cycles. This study shows that changes in boreal summer insolation amplitude altered westerly wind patterns and regional hydroclimate, affecting ice sheet growth and likely contributing to the shift to 100-kyr glacial cycles during the Mid-Pleistocene Transition.